1. Field of the Invention
The present invention relates to an ignition timing control system for an internal-combustion engine, for controlling the ignition timing in proportion to the magnitude of knocking of the internal-combustion engine.
2. Description of the Prior Art
A known ignition timing control system for suppressing the generation of knocking in an internal-combustion engine detects a signal which is specific to knocking by sensing the vibration of the engine or the internal pressure of the cylinders and delays the ignition timing on the basis of the detected signal.
Generally, when knocking occurs, a high-frequency vibration of several kilohertz is generated in the internal-combustion engine. This high-frequency vibration overlaps with the vibration of the engine and the pressure variation within the combustion chamber. Accordingly, a knocking signal can be obtained by converting the detected vibration of the engine and the pressure variation within the combustion chamber into an electric signal and then discriminating the specific high-frequency component corresponding to the knocking by means of a frequency filter or the like.
First a conventional ignition timing control system for an internal-combustion engine will be described in connection with FIGS. 1 and 2.
Referring to FIG. 1 showing a block diagram of a conventional ignition timing control system, the ignition timing control system comprises a knocking sensor 1 for detecting vibration and or variations in pressure within the cylinder resulting from the generation of knocking in the internal-combustion engine and for converting the detected signal into an electric signal, a knocking detector 2 for discriminating a specific knocking signal component by processing the output signal provided by the knocking sensor 1 through an internal frequency filter and an internal comparator of a predetermined threshold level and for providing a knocking detection signal, an integrator 3 for raising the output voltage level by integrating the pulses of the knocking detection signal over a predetermined time constant and for lowering the output voltage level over a predetermined time constant when no knocking detection signal is applied thereto, a reference ignition timing signal generator 4 for generating a predetermined reference ignition timing signal corresponding to the revolving speed and the load conditions of the engine, a phase delay controller 5 for delaying the phase of the reference ignition timing signal in proportion to the output voltage level of the integrator 3 and a transistor T.sub. R for interrupting current supply to an ignition coil C.sub.I in synchronism with the output signal of the phase delay controller 5.
The conventional ignition timing control system as described above operates in the following manner.
Referring to FIG. 2, when knocking occurs in the internal-combustion engine, the knocking sensor 1 provides a knocking signal as shown in FIG. 2-(a). When knocking is generated, an oscillatory wave form of an amplitude far greater than that of the oscillatory waveform of the noise component is provided. The magnitude of knocking is represented by the magnitude of the amplitude of the oscillatory waveform, namely, weak knocking by the waveform in the section A of FIG. 2, while strong knocking by the waveform in the section B of FIG. 2. Upon the reception of the knocking signal, the knocking detector 2 compares the waveform of the knocking signal with a predetermined threshold level, which is greater than the noise component as shown in FIG. 2-(b), by means of the internal comparator to discriminate a specific knocking signal included in the knocking signal and then provides a knock detection signal of a pulse train as indicated in FIG. 2-(c). Upon the reception of the knocking detection signal, the integrator 3 integrates pulse train. That is, the integrator 3 integrates the pulse train of the knocking detection signal over a predetermined time constant to raise the output voltage level and when no knocking detection signal is applied thereto, the integrator 3 lowers the output voltage level over a predetermined time constant. The integral of the pulse widths of the knocking detection signal thus obtained correlates strongly with the magnitude of the knocking as in FIG. 2-(d). On the other hand, the reference ignition timing signal generator 4 generates a reference ignition timing signal corresponding to the revolving speed and the load conditions of the engine. Upon the reception of the reference ignition timing signal, the phase delay controller 5 delays the phase of ignition timing in proportion to the integral of the pulse train of the knocking detection signal provided by the integrator 3 to control the phase of ignition timing in proportion to the magnitude of the knocking. The output pulse of the phase delay controller 5 is applied to the base of the transistor T.sub.R. Then, the rising and the breaking of the pulse are used for switching on and switching off the transistor T.sub.R, whereby current supply to the ignition coil C.sub.I is interrupted intermittently. When current supply is interrupted the engine is ignited.
Incidentally, the condition of knocking varies depending not only on comparatively stable conditions such as the temperature and the humidity of the suction air, the air fuel ratio and ignition timing, but also on various conditions which change subtly every combustion, such as the internal condition of the combustion chamber and flaming speed, therefore, knocking phenomena of the same mode do not necessarily occur every combustion even if the engine is in a stabilized steady state operating condition. Accordingly, it is possible that comparatively strong knocks occur suddenly or occasionally even during the steady state operation of the engine. When strong knocks occur suddenly or occasionally, according to a phase control method in which the phase of ignition timing is delayed in proportion to the magnitude of knocking, the phase of ignition timing is temporarily delayed excessively for a single strong knock, which possibly affect the output of the engine and the exhaust temperature unfavorably. Accordingly, such a problem can be solved if the controlled phase delay for coping with above-mentioned occasional strong knocks can be restricted within a range which will not affect unfavorably the operation of the engine, when a strong knock occurs.